Insulin is a polypeptide hormone that is produced in the pancreatic β-cells of normal (non-diabetic) individuals. Human insulin is a 51 amino acid polypeptide hormone with a molecular weight of about 5800 daltons. The insulin molecule is composed of two peptide chains (an A and a B chain) containing one intrasubunit and two intersubunit disulfide bonds. The A chain is composed of 21 amino acids while the B chain is composed of 30 amino acids. The two chains of insulin form a highly ordered structure with several α-helical regions in both the A and the B chains. Interestingly, the isolated chains of insulin are inactive. In solution, insulin can exist as a monomer or as a dimer or as a hexamer. Insulin is hexameric in the highly concentrated preparations used for subcutaneous therapy but becomes monomeric as it is diluted in body fluids. Insulin is necessary for regulating carbohydrate metabolism by reducing blood glucose levels; a systemic deficiency of insulin causes diabetes. The survival of diabetic patients depends on the frequent and long-term administration of insulin to maintain acceptable blood glucose levels.
Current insulin formulations possess deficiencies that can lead to serious medical complications in the treatment of diabetes. For instance, the standard zinc insulin preparation most commonly used by diabetics exists as a suspension of microcrystals of inactive hexameric insulin. Dissolution of the microcrystals followed by dissociation of the hexamer into the active monomer form can lead to delayed and individually variable absorption of insulin into the bloodstream (F. Liu, et al., Bioconjugate Chem., 8, 664-672 (1997); T. Uchio, et al., Adv. Drug Del. Rev., 35, 289-306 (1999); K. Hinds, et al., Bioconjugate Chem., 11, 195-201 (2000). Formulations of insulin also suffer from physical instability due to the tendency of insulin to form fibrils and insoluble precipitates. Precipitation is especially problematic for formulations intended for use in insulin pumps. Formulated insulin is also prone to chemical degradation, e.g., non-enzymatic deamidation and formation of high molecular weight transformation products such as covalent insulin dimers (Brange, J., et al., Pharm. Res., 9, 715-726 (1992); Brange, J., et al., Pharm. Res., 9, 727-734 (1992). There is significant evidence that the incidence of immunological responses to insulin may result from the presence of these covalent aggregates of insulin (Robbins, D.C., et al., Diabetes, 36, 838-841 (1987). Moreover, even highly purified human insulin is slightly immunogenic. (Kim, ibid.)
Apart from the formulation instability problems noted above, there are also numerous drawbacks associated with current insulin therapies from an administration standpoint. Insulin is most commonly administered by subcutaneous injection, typically into the abdomen or upper thighs. Insulin may also be administered intravenously or intramuscularly. In order to maintain acceptable blood glucose levels, it is often necessary to inject insulin at least once or twice per day, with supplemental injections of insulin being administered when necessary. Aggressive treatment of diabetes can require even more frequent injections, where the patient closely monitors blood glucose levels using a home diagnostic kit. The administration of insulin by injection is undesirable in a number of respects. First, many patients find it difficult and burdensome to inject themselves as frequently as necessary to maintain acceptable blood glucose levels. In fact, many Type 2 patients avoid going on insulin for years because of needle phobia. Such reluctance can lead to non-compliance, which in the most serious cases can be life-threatening. Moreover, systemic absorption of insulin from subcutaneous injection is relatively slow, frequently requiring from 45 to 90 minutes, even when fast-acting insulin formulations are employed. Thus, it has long been a goal to provide alternative insulin formulations and routes of administration which avoid the need for self-injection and which can provide rapid systemic availability of insulin.
Numerous non-injectable formulation types such as oral and nasal have been explored, however, no commercially viable oral or nasal-based delivery system for insulin has been developed as a result of these efforts (Patton, et al., Adv. Drug Delivery Reviews, 1, 35 (2-3), 235-247 (1999)), mainly due to very low and variable bioavailability (Hilsted, J., et al., Diabetologia 38, 680-684, (1995)). Although bioavailability can be increased with absorption enhancers, these agents can damage the mucosa.
However, inhaleable formulations of insulin have been developed which appear to be quite promising in overcoming many of the problems noted above. For example, U.S. Pat. No. 5,997,848 (Patton, et al., Inhale Therapeutic Systems, Inc.) describes dry powder formulations of insulin which (i) are chemically and physically stable at room temperature, and (ii) when inhaled, are rapidly absorbed through the epithelial cells of the alveolar region into the blood circulation. The rapid-acting insulin formulations and methods described therein avoid the need for burdensome self-injections, and have been shown in three month human efficacy studies to provide equivalent glucose control in Type I and Type II insulin-dependent diabetics when compared to subcutaneous injection (Patton, et al., Adv. Drug Delivery Reviews, 1, 35 (2-3), 235-247 (1999)). The dry powder insulin formulations described by Patton, et al., while overcoming the problems of formulation instability and patient non-compliance, still require frequent (e.g., mealtime) inhalations of insulin for effective control of glucose levels. Moreover, a typical insulin dosing regime of this type, based on rapid acting inhaleable insulin, still requires a single injection of long-acting insulin at bedtime for Type I and some Type II diabetics. Thus, there still exists a need for active, soluble, stable forms of insulin that require less frequent dosing, i.e., long-acting insulin formulations, preferably administrable by inhalation.
Long-acting insulin formulations are ideally characterized as having a very slow onset and a prolonged, relatively flat peak of action. Current long acting injectable insulin formulations, e.g., ultralente (extended insulin zinc suspension) and protamine zinc insulin suspension, are very unsatisfactory. These formulations tend to peak rather than provide a low basal concentration of insulin, are unpredictable, and typically exhibit a duration of action of no longer than about a day. The long half-life of ultralente insulin makes it difficult to determine the optimal dosage range, and protamine zinc insulin is rarely used because of its unpredictable and prolonged course of action (Goodman & Gilman, “The Pharmacological Basis of Therapeutics, Ninth Ed., Hardman and Limbird, eds, 1996, p. 1500). Other long-acting injectable formulations which have been explored unsuccessfully include albumin-bound insulin and cobalt-insulin hexamer formulations (Hoffman, A., Ziv E., Clin. Pharmacokinet, 33(4):285-301 (1997)).
A number of long-acting pulmonary insulin formulations have also been explored. These include liposomes containing a large excess of lipid relative to insulin (Liu. F-Y, et al., Pharm. Res. 10, 228-232, (1993)), porous poly(lactic acid-co-glycolic acid) (PLGA) insulin particles (Edwards, D. A., et al., Science 276(5320), 1868-1871 (1997)), nebulized PLGA nanospheres (Kawashima, Y., et al., J. Controlled Release, 62(1-2): 279-287 (1999)) and phospholipid/protamine insulin formulations (Vanbever, R., et al., Proc. Control Rel. Bioact. Mater. 25, 261-262 (1998)). Unfortunately, all of these formulations have proven unsatisfactory, due to either low bioavailabilities when administered in rats, or due to formulation insufficiencies. Thus, a long-felt need exists for optimized long-acting insulin formulations that are bioactive, physically and chemically stable, water-soluble, and preferably monomeric. Ideally, such formulations will preferably be suited for pulmonary administration.